A technique of casting which employs a plastic pattern consumed by the molten metal during the casting process, appears to offer benefits with respect to a more streamlined foundry technique. The plastic pattern, which has been found to possess the greatest advantages, is that of a low density polystyrene expanded plastic (low density expanded closed-cell polymeric material shall be defined herein as having a density of 3.0 lbs/ft.sup.3 or less). This type of plastic is easily distorted by pressure and can be easily changed in shape by manual handling. If such patterns are to be employed in a casting process for making relatively complex parts, the integrity of the outer surface of such expanded plastic must be maintained so as not to result in inexact castings. This problem is compounded when the casting pattern is formed in multiple parts and subsequently joined to form a completed pattern. Several patterns may be integrated into a single cluster or tree to permit the simultaneous pouring of several castings; this will require joining of not only the split patterns but the many patterns and gating elements. Moreover, the singular split pattern may have a parting plane which is stepped and too complex for heat sealing; this type of split pattern is necessitated in castings such as for automotive crankshafts and complex engine housings.
Joining plastic members together has been carried out by many modes including heat sealing. However, heat sealing requires that a significant zone of each member be melted; the control of the zone of heating to a precise surface to be mated is difficult. Moreover, nonuniform heat distribution throughout the plastic may result in distortion in certain regions that is undesirable.
The joining together of plastic parts by a totally mechanical method, utilizing pressure and movement, has been long sought. Such a joining method would be preferable because of the hope that the integrity of the plastic configuration would be maintained before and after the joining sequence. High density plastics have been successfully sealed together by the use of a very high clamping pressure and accompanying relative vibration between the parts. However, to date, low density plastics have not been successfully joined by a mechanical method. Various modes of interlocking low density plastics have been employed, but because of the fragile and distortable nature of such plastics, they have proved to be inadequate. For example, interlocking a tongue and a groove portion of two plastic parts requires application of a distortable pressure on certain locking surfaces which leads to a change in shape of the plastic parts (See U.S. Pat Nos. 3,635,280 and 3,695,340). But more importantly the crevices formed at the interior locking surfaces create a path for chemical washes or refractory to seep inwardly of the pattern and generate defects in the casting when the pattern is consumed, particularly when the washes are a refractory material. If such parts are to be used as a mold, an inexact casting is the result. The exterior configuration of the parts when joined together must remain unchanged between the before and after conditions.